Arrangement and method for determining the spatial distribution of magnetic particles

ABSTRACT

A system for determining the spatial distribution of magnetic particles in an examination area. Magnetic field generator generates a spatially inhomogeneous gradient magnetic field with at least one region with a low field strength in which the magnetization of the particles is in a state of non-saturation, whereas they are in a state of saturation in the remaining region. By an arrangement to shift the area with a low field strength within the examination area, a change in the magnetization of the magnetic particles is brought about which can be detected from outside by a detector. At least one of the magnetic field generator, the arrangement and the detector are arranged at least partially on a medical instrument.

This application is a continuation of prior U.S. patent application Ser.No. 11/721,565, filed Jun. 13, 2007, which was the National Stage ofInternational Application No. PCT/IB2005/054248, filed Dec. 14, 2005,which claims the benefit of European Patent Application (EPO) No.04106838.8, filed Dec. 22, 2004, which is a continuation-in-part ofprior U.S. patent application Ser. No. 10/270,991, filed Oct. 15, 2002,which claims the benefit of German Patent Application No. 10151778.5,filed Oct. 19, 2001.

The invention relates to an arrangement for determining the spatialdistribution of magnetic particles in an examination area using medicalinstruments and also to a method of determining the distribution.Furthermore, the invention also relates to medical instruments and tothe use thereof in the abovementioned arrangement.

Such an arrangement and such a method are already known from DE10151778.In the method described therein, a spatially inhomogeneous magneticfield is generated, having at least one region with a low fieldstrength, in which the magnetization of the particles is in a state ofnon-saturation, whereas they are in a state of saturation in theremaining region. By shifting the region with a low field strengthwithin the examination area, a change in magnetization is brought aboutwhich can be detected from outside and contains information about thespatial distribution of the magnetic particles in the examination area.An arrangement for carrying out the method is also disclosed. Theexamination area is surrounded by a few coil arrangements, by means ofwhich the inhomogeneous magnetic field is generated, the shift in themagnetic field regions is brought about and signals are detected. Thesignals are then evaluated.

It is an object of the invention to develop an improved arrangement.

This object is achieved as claimed in claim 1 by an arrangement fordetermining the spatial distribution of magnetic particles in anexamination area, comprising

a) magnetic field means for generating a magnetic field with a spatialcourse of the magnetic field strength such that there is in theexamination area a first part-region with a low magnetic field strengthand a second part-region with a higher magnetic field strength,

b) change means for changing the spatial position of the twopart-regions in the examination area, so that the magnetization of theparticles changes locally,

c) detection means for detecting signals which depend on themagnetization in the examination area that is affected by the change inspatial position,

d) evaluation means for evaluating the signals in order to obtaininformation about the spatial distribution of the magnetic particles inthe examination area, wherein the magnetic field means or the changemeans or the detection means or a combination of these means arearranged at least partially on a medical instrument.

With regard to the description of the invention, reference will be madeto the following documents by the same Applicant, which are hereby fullyincorporated by way of reference into the present text:

Ax1: German patent application DE10151778A1 bearing the title “Verfahrenzur Ermittlung der räumlichen Verteilung magnetischer Partikel”,

Ax2: European patent application bearing the application numberEP03101014.3 and the title “Verfahren and Gerät zur Beeinflussungmagnetischer Partikel”,

Ax3: German patent application DE10238853A1 bearing the title “Verfahrenzur lokalen Erwärmung mit magnetischen Partikeln”.

The magnetic field means, change means and detection means and the modesof action thereof are described in general in documents Ax1 and Ax2, sothat these means will be discussed here only with regard to the presentinvention. For further details regarding said means, reference should bemade to the aforementioned documents.

As is known from the aforementioned documents, a gradient magnetic fieldhaving a first part-region which is for example a spatially coherentregion is formed by the magnetic field means. In this part-region, themagnetic field is so weak that the magnetization of the particlesdiffers to a greater or lesser extent from the external magnetic field,that is to say is not saturated. In the second part-region (that is tosay in the rest of the examination area outside the first part or inthat region which surrounds the first part-region), the magnetic fieldis strong enough to keep the particles in a state of saturation.

According to document Ax1, the magnetic field means consist for exampleof a Maxwell coil arrangement, of a coil and a permanent magnet, or oftwo permanent magnets opposite one another with identically poled ends.In addition, coil arrangements are also known from the document Ax2 inwhich the examination area does not lie within the magnetic field meansbut rather next to them. In all these arrangements, an inhomogeneousmagnetic field or a gradient magnetic field is produced, in which afirst part-region is formed which has a lower magnetic field strength orno magnetic field strength at all compared to its surroundings. Such amagnetic field is shown for example in documents Ax1 to Ax3, forinstance in FIG. 2 in document Ax1.

According to the invention, the magnetic field means may be arranged atleast partially on a medical instrument. As a result, it is possible toplace the region with a low magnetic field strength in the vicinity ofthe medical instrument, in order to determine the distribution of themagnetic particles in the vicinity of the medical instrument. As aresult, a higher gradient of the magnetic field is produced in thevicinity of the region with a low field strength, and this results in animproved resolution. Moreover, when the medical instrument is moved, theposition of the region with a low field strength will not move or willhardly move with respect to the medical instrument, so that the regionwith a low magnetic field strength essentially follows the movement ofthe medical instrument. As a result, the distribution of the magneticparticles can always be determined in the vicinity of the medicalinstrument.

In the known arrangements from documents Ax1 to Ax3, the position of theregion with a low magnetic field strength is predefined by thecorresponding magnetic field means in the basic state, that is to saywithout any action by the change means. If the examination object (e.g.the patient) moves or shifts relative to the examination area during theexamination, only a distorted or even completely incorrect assignment ofthe various positions of the region with a low field strength to thecorresponding regions of the patient is possible. As a result, movementartifacts arise when determining the spatial distribution of themagnetic particles, as is also known for example from nuclear spintomography. These movement artifacts can be reduced or avoided by atleast partially arranging the magnetic field means on the medicalinstrument.

If the magnetic field means consist for example of a Maxwell coilarrangement, one of the two coils may be arranged on the medicalinstrument. If the magnetic field means include at least one permanentmagnet, this may be arranged on the medical instrument as claimed inclaim 2. The arrangement of a permanent magnet on the medical instrumentis usually simple to achieve. In particular, unlike a coil, there is noneed for any power supply or for lines to be guided out of the medicalinstrument. The embodiment as claimed in claim 3 corresponds to asimilar arrangement, as disclosed in document Ax2. If the magnetic fieldmeans consist only of the components described therein, the magneticfield means may be arranged not just partially but rather completely onthe medical instrument. As a result, the components surrounding theexamination area are reduced.

As shown by way of example, magnetic field means may in general consistof one or more components. Therefore, in this connection, a partialarrangement of the magnetic field means on the medical instrument meansthat at least one component of the magnetic field means is arranged onthe medical instrument.

The change in spatial position of the two part-regions may be broughtabout by means of various change means, as discussed for example indocuments Ax1 or Ax2. On the one hand, coil arrangements of the magneticfield means may be used for this purpose if they are operated with an ACcurrent in addition to a DC current. On the other hand, separate changemeans may also be used, for example by using a dedicated coilarrangement to generate a temporally variable magnetic field which issuperposed on the gradient magnetic field. According to the invention,at least one such coil arrangement may be arranged on the medicalinstrument. As a result, during a periodic change in the position of thetwo part-regions, a much higher frequency can be used than in the knownarrangements. This is because, in the arrangement of the change meanswhich is known from the documents Ax1 to Ax3, the temporally variablemagnetic fields are applied to a relatively large area of theexamination object (patient). Possible heating of the examination objectdepends approximately on the product of amplitude and frequency of thetemporally variable magnetic field. By arranging change means on themedical instrument, temporally variable magnetic fields are applied onlylocally in the vicinity of the medical instrument. As a result, theoverall and local heating of the examination object is considerablyreduced.

As shown by way of example, the change means may in general also consistof one or more components. In this connection, therefore, a partialarrangement of the change means on the medical instrument means that atleast one component of the change means is arranged on the medicalinstrument.

If coil arrangements are at least partially used in the change means,these are operated with high frequencies, depending on the operatingmode. The coil arrangements may heat up on account of the electricalresistance. Such a heating beyond a specific level may be undesirable inthe case of a coil arrangement arranged on the medical instrument. Theheating of a coil operated with high frequencies can be reduced asclaimed in claim 4. As is known, the current flowing through thewindings or the number of windings of the coil can be reduced by using acore, as a result of which the heating is reduced. The heat lossescaused by the magnetic reversal in the core are minimized by itssoft-magnetic properties, particularly if it has a magnetizationcharacteristic which is as linear as possible within the range of themagnetic field strengths acting thereon.

According to documents Ax1 and Ax2, the signals coming from the magneticparticles can be detected by at least one coil or coil arrangement asdetection means. According to the invention, this coil or coilarrangement may be arranged on the medical instrument. As a result, thesignals can be detected with a considerably improved signal-to-noiseratio if the region with a low magnetic field strength is located in oris shifted into the vicinity of the detection means. As claimed in claim5, use may also be made of a number of coils or coil arrangements havingdifferent directions of action. By way of example, as is known, a flatconductor loop can detect signals particularly well if the changingmagnetic field stands vertical on the surface formed by the conductorloop. A direction of action of a coil or coil arrangement is thereforeto be understood as meaning that direction in which a changing magneticfield acts on the coil or coil arrangement, with regard to maximumsignal detection.

As shown by way of example, the detection means may in general alsoconsist of one or more components. In this connection, therefore, apartial arrangement of the detection means on the medical instrumentmeans that at least one component of the detection means is arranged onthe medical instrument.

Within the context of this invention, a medical instrument is to beunderstood as meaning any article which can be used by a doctor or otherstaff for medical purposes, for example examinations or treatments. Onthe one hand, this is to be understood as meaning articles which arepassed over the object to be examined and are placed on the patient'sskin for example in the form of a scanning head. This term also includesscanning heads for example. Using such medical instruments, it is thenpossible to create images for example of blood vessels below the skin,using the invention and the methods described in documents Ax1 and Ax2.On account of the laws of physics, it is expected that signals worthy ofevaluation can be generated and detected only up to a certainpenetration depth in the examination object. If coils are used asmagnetic field means, this penetration depth is proportional for exampleto the central area of these coils.

Furthermore, as claimed in claim 6, invasive medical instruments such asinstruments for minimally invasive operations or a catheter as claimedin claim 7 also fall under the term “medical instrument”. This term isalso to be understood as meaning probes which can be inserted into thegullet, stomach, intestine, ear or other points of the human or animalbody. This list is given by way of non-limiting example.

In order to be able to determine its position more easily, a marker isarranged on the medical instrument as claimed in claim 8. Such markersare disclosed in the document Ax4 bearing the title “Markers forposition determination using magnetic methods”, which was filed as apatent application with the European Patent Office on the same day andby the same Applicant as the present invention. Said patent applicationis hereby fully incorporated by way of reference.

By modifying the change means, the arrangement as claimed in claim 9 canalso be used to implement a local hyperthermia, as disclosed in documentAx3.

As claimed in claims 10 to 13, use may be made of known medicalinstruments comprising coils, such as catheters from the field ofnuclear spin tomography for example.

The method as claimed in claim 14 is based on the methods disclosed indocuments Ax1, Ax2 and Ax3.

The invention will be further described with reference to examples ofembodiments shown in the drawings to which, however, the invention isnot restricted. The medical instrument described here is a catheter,although in principle other medical instruments may be used instead.

FIG. 1 shows a first catheter according to the invention.

FIG. 2 shows a second catheter according to the invention.

FIG. 3 shows the course of a first gradient magnetic field.

FIG. 4 shows the course of a second gradient magnetic field.

FIG. 1 schematically shows the tip of a catheter 8. As is known, acatheter forms a thin hose-like line, through the interior of which forexample a guidewire runs or liquids (such as contrast agents) are passedto the catheter tip and can exit through the opening 10.

In order to obtain information about the spatial distribution ofmagnetic particles in the examination object (here the patient) whichare located in the vicinity of the catheter, coils and coil pairs arelocated on the catheter 8, the magnetic fields of which also passthrough a region in front of the catheter tip. The coils mentioned beloware shown only schematically as circles, so that the supply lines forexample are not shown for the sake of better clarity. A first coil paircomprises the two windings 13 a and 13 b which surround one anothercoaxially and during operation are flowed through by currents inopposite directions, a common axis 60 of which windings runs more orless along an axis of the catheter 8. The gradient magnetic fieldgenerated thereby is shown and described in FIGS. 2a and 2b of thedocument Ax2. The position of the field-free point or of the region witha low field strength is selected such that it is located in front of theopening 10 of the catheter 8. Starting from this field-free point, thestrength of the magnetic field increases in all three spatial directionas the distance from the field-free point increases.

In order to position the field-free point on the common axis, variousparameters of the arrangement may be changed. If the current intensityof the current flowing through the winding 13 a is increased or thecurrent intensity of the current flowing through the winding 13 b isreduced, the field-free point is displaced in the direction of thecatheter. If, on the other hand, the current intensity of the currentflowing through the winding 13 a is reduced or the current intensity ofthe current flowing through the winding 13 b is increased, thefield-free point is displaced in the opposite direction. Moreover, theposition, in particular the starting position, of the field-free pointcan be affected by changing the diameter of the windings 13 a and 13 b.Furthermore, it must be ensured by virtue of the dimensioning of thecoil arrangement that the spatial size of the region with a higher fieldstrength is sufficiently large. This means that, in this region, thereshould be magnetic particles whose signals could in principle bedetected by the detection means described below but which in fact do notgenerate any signal and therefore are kept in a state of magneticsaturation. If the magnetic particles are far away from the detectionmeans such that their signals are detected only weakly or not at all bythe detection means, they moreover need no longer lie in the region witha higher magnetic field strength.

The size of the region with a low field strength (shown at reference 301in FIG. 2b of document Ax2) which determines the spatial resolution ofthe device depends on the one hand on the strength of the gradient ofthe gradient magnetic field and on the other hand on the size of themagnetic field required for saturation. For deeper consideration,reference should be made to documents Ax1 and Ax2.

If one or more further magnetic fields are superposed on the gradientmagnetic field in the zone of action, the field-free point or the regionwith a low field strength is then displaced along this superposedmagnetic field, wherein the size of the displacement increases with thestrength of the superposed magnetic fields. The superposed magneticfields have different directions and may be temporally variable.

In order to generate these temporally variable magnetic fields for anydirection in space, three further coil arrangements are provided aschange means. A coil 14 generates a magnetic field which runs in thedirection of the coil axis of the coil pair 13 a, 13 b. In principle,the effect that can be achieved by means of this coil pair can also beachieved by superposing currents of the same direction on the currentsof opposite direction in the coil pair 13 a, 13 b, as a result of whichthe current decreases in one coil pair and increases in the other coilpair. However, it may be desirable if the temporally constant gradientmagnetic field and the temporally variable vertical magnetic field aregenerated by separate coil pairs.

In order to generate magnetic fields which run spatially perpendicularto the common axis of the coils 13 a and 13 b and/or perpendicular tothe catheter axis, two further coils pairs are provided, comprising thewindings 15 a, 15 b and 16 a, 16 b. The winding 16 b is not shown sinceit is arranged on the underside of the catheter, which is not visible.The windings 15 a and 15 b and the windings 16 a and 16 b arerespectively arranged in an identical manner on the outer surface of thecatheter 8 and lie opposite one another. The common axis of the coilpair comprising the windings 15 a and 15 b is perpendicular to thecommon axis of the coil pair 16 a and 16 b and the two axes of the coilpairs are in each case perpendicular to the axis of the catheter 8.During operation, a magnetic field forms between the two windings of acoil pair, the field lines of which magnetic field run on the one handalmost in a straight line through the catheter 8. On the other hand,they run in a curved manner around the catheter 8, wherein they alsopass through the field-free point or the region with a low fieldstrength in front of the catheter tip 8 with a component perpendicularto the catheter axis.

The shape of the windings may also be different in order to optimize therespective curved magnetic field. It is also conceivable for the reasonsmentioned above to arrange a soft-magnetic core (not shown) inside therespective coils.

Finally, FIG. 1 shows a further coil 17 which serves to detect signalsgenerated in the zone of action. In principle, any of thefield-generating coil pairs 13 to 16 could also be used for thispurpose. However, when use is made of a special coil, a more favorablesignal-to-noise-ratio is obtained, particularly if a number of receivingcoils (not shown) are used. In addition, the coil may be arranged andconnected in such a way that it is decoupled from the other coils. If,for example, three receiving coils are fitted on the catheter, theirdirections of action may lie at an angle of 90° with respect to oneanother. As a result, signals are detected from all directions aroundthe catheter tip. In addition, it is also possible to fit other externalreceiving coils (not shown) next to the examination object.

By virtue of the design of the catheter shown in FIG. 1, the position ofthe region with a low magnetic field strength relates to the catheterand no longer to the examination area or to the external components asdescribed in documents Ax1 and Ax1 [sic]. In the basic state, therefore,the position of the region with a low field strength changes within theexamination object only when there is a relative movement betweencatheter and examination object. If, during the time of signaldetection, the catheter is stationary with respect to the region of thepatient from which the signals are to be detected, the patient can movewithout this giving rise to movement artifacts. If images of the innerwall of an artery or of a coronary vessel are to be created by means ofthe catheter, for example, it is expected that even the complex movementof the heart will not lead or will lead only slightly to movementartifacts.

It is also possible, depending on the application, not to fit all of thecoils shown in FIG. 1 to the catheter 8. By way of example, the coils14, 15 a, 15 b, 16 a and 16 b may be omitted if external coils asdescribed in Ax1 and Ax2 are used to shift the region with a low fieldstrength.

In respect of the changing of the region with a low field strength, thedetection of the resulting signals and the evaluation of said signals,reference should be made to documents Ax1 and Ax2. In addition, heatingof magnetic particles is also possible, as described in document Ax3.Heating takes place here in the vicinity of the tip of the catheter.Prior to heating, the region to be heated can be displayed by means ofthe above-described method and a user can define the region to beheated.

In the catheter 8, it is also possible, by virtue of a different designand arrangement of the magnetic field means, to define the position ofthe field-free point or of the region with a low field strength so thatit is not in front of but rather next to the tip of the catheter 8. Thisis useful for example when images of regions which are mainly locatednext to the catheter 8 are to be created. The coils or coil arrangementsshown in detail in FIG. 1 may then possibly have different shapes or beoriented differently and as a result be arranged differently on thecatheter 8. However, their function does not change.

FIG. 2 shows a catheter 8 a which essentially corresponds to thecatheter 8 of FIG. 1. With the exception of the coils 13 a and 13 b, thecatheter 8 a contains all the components of the catheter 8, but theseare not shown for the sake of clarity. In order to generate the gradientmagnetic field, a bar-shaped permanent magnet 25 is fitted on thecatheter 8 a by a clip 26, one of the poles of said magnet lying at theend of the tip of the catheter 8 a. When an external magnetic fieldwhose field lines run more or less parallel to the axis of the catheteracts on said catheter, the field line course shown schematically in FIG.3 is obtained. In FIG. 3, the external magnetic field is generated by acoil pair comprising the coils 30 and 31, wherein the coils 30 and 31are arranged for example around the examination object. Instead of thecatheter 8 a, only the permanent magnet 25 is shown here. The left endof the permanent magnet 25 in FIG. 3 corresponds to the end of thepermanent magnet 25 which faces the tip of the catheter as can be seenin FIG. 2.

By virtue of superposition with the magnetic field of the permanentmagnet 25, a region 27 with a low magnetic field strength is obtained infront of the tip of the catheter 8 a. If the catheter 8 a with thepermanent magnet 25 is displaced within the virtually homogeneous regionof the external magnetic field, the region 27 accordingly movestherewith, without changing its position with respect to the catheter.The displacement of the region 27 and the detection of signals takeplace in the same manner as described for the catheter 8.

Compared to the catheter 8, the catheter 8 a has fewer components.However, the region 27 is displaced with respect to the catheter 8 awhen the catheter 8 a or the permanent magnet 25 rotates with respect tothe illustrated position in such a way that the field lines of theexternal magnetic field no longer run parallel to the axis of thepermanent magnet 25 or catheter 8 a. This shift in position can becompensated for example by corresponding activation of the other coilsor can be taken into account during signal evaluation. Anotherpossibility is to likewise change the direction of the external magneticfield in a manner corresponding to the rotation of the permanent magnet25. To this end, for example, further external magnetic fields withdifferent directions may be superposed on the external magnetic field.The direction in which the catheter 8 a or the permanent magnet 25rotates may for example be determined by three orthogonal receivingcoils arranged next to the examination object.

As an alternative to the illustrated bar-shaped permanent magnet 25, usemay also be made of a ring-shaped permanent magnet which surrounds thetip of the catheter 8 a or is embedded therein. Instead of a permanentmagnet, use may in general also be made of a coil 26, wherein the courseof the field lines shown in FIG. 4 is obtained.

The catheter 8 shown in FIG. 1 furthermore has a marker 20. Such markersand their function are described in detail for example in document Ax4,so that no further details are given at this point. If use is made of amarker which itself has magnetic properties (since it contains magneticparticles for example), it interferes with the other components arrangedon the catheter 8. This is because on the one hand it distorts thegradient magnetic field and on the other hand it possibly also generatesharmonic frequencies of the basic frequency, by means of which theregion with a low field strength is shifted into its vicinity. Themarker 20 should therefore be arranged at a sufficiently large distancefrom these components. Alternatively, the marker may also be configuredsuch that its magnetic properties differ from the magnetic properties ofthe magnetic particles located in the surroundings of the catheter 8.Such differences may lie for example in the course of the magnetizationcurve (steepness, hysteresis). The signal coming from the marker thenhas a different spectral composition from the signals coming from themagnetic particles. In the case of a very large hysteresis, the signalcoming from the marker may even almost disappear.

In the catheter 8 shown in FIG. 1, the marker may also be used tocalibrate the position of the region with a low field strength withrespect to the currents flowing through the coils. This is possiblesince the geometric position of the marker with respect to the coils isknown. If the magnetic field means, in this case the coil paircomprising the two windings 13 a and 13 b, are not fitted on thecatheter, the position of the catheter in the examination area can bedetermined by means of the marker.

The invention claimed is:
 1. A system for determining spatialdistribution of magnetic particles in an examination area, the systemcomprising: an invasive medical instrument; a first coil device havingfirst and second coils configured to generate a magnetic field in theexamination area including a first region with a low magnetic fieldstrength and a second region with a higher magnetic field strength; asecond coil device configured to generate a spatially variable magneticfield in the examination area to change a spatial position of the firstregion responsive to movement of the invasive medical instrument therebychanging a magnetization of the magnetic particles local to the invasivemedical instrument with the first region substantially followingmovement of the invasive medical instrument and substantiallymaintaining a position of the first region with respect to the invasivemedical instrument; a third coil device configured to detect the changein the magnetization of the magnetic particles in the examination areaaffected by the change in the spatial position of the first region; anda processor configured to determine a spatial distribution of themagnetic particles in the examination area, wherein the spatialdistribution is determined based on the detected change in magnetizationof the magnetic particles, wherein at least one of the first, second andthird coil devices are arranged on the invasive medical instrument. 2.The system as claimed in claim 1, wherein the first coil device isarranged on the invasive medical instrument and comprises at least oneof a coil arrangement and a permanent magnet.
 3. The system as claimedin claim 1, wherein the first coil device is arranged on the invasivemedical instrument and comprises at least two coils arranged one insideanother that during operation are flowed through by currents that flowrespectively in an opposite direction in one of the at least two coilsas compared to another one of the at least two coils.
 4. The system asclaimed in claim 1, wherein the second coil device is arranged on theinvasive medical instrument and comprises a coil arrangement with amagnetic core.
 5. The system as claimed in claim 1, wherein the thirdcoil device is arranged on the invasive medical instrument and comprisesa plurality of coils or coil arrangements, wherein directions of actionof the plurality of coils or coil arrangements are at an angle to oneanother.
 6. The system as claimed in claim 1, wherein the invasivemedical instrument is a catheter having a tip and the at least one ofthe first, second and third coil devices are arranged on or in avicinity of the catheter tip.
 7. The system as claimed in claim 6,wherein the invasive medical instrument has an opening located at thetip and the second coil device is arranged on the invasive medicalinstrument and wherein the first region with the low magnetic fieldstrength is located in front of the opening.
 8. The system as claimed inclaim 7, wherein the first coil device is arranged on the invasivemedical instrument and comprises at least two coils arranged coaxiallyone inside another each having a common axis that runs substantiallyalong an axis of the invasive medical instrument.
 9. The system asclaimed in claim 1, wherein the invasive medical instrument comprises amarker.
 10. The system as claimed in claim 1, wherein the second coildevice is further configured to change the spatial position of the firstand second regions in a target area to heat up the target area.
 11. Thesystem as claimed in claim 1, wherein the at least one of the first, thesecond, and the third coil devices are configured for a medical purpose.12. The system as claimed in claim 11, wherein the invasive medicalinstrument is a catheter.
 13. The system as claimed in claim 1, whereinthe invasive medical instrument is a catheter.
 14. A method ofdetermining the spatial distribution of magnetic particles in anexamination area, the method comprising acts of: introducing an invasivemedical instrument including at least one of first, second and thirdcoil devices into the examination area, wherein the first coil devicegenerates a magnetic field in the examination area including a firstregion with a low magnetic field strength and a second region with ahigher magnetic field strength; moving the invasive medical instrumentin the examination area; generating a spatially variable magnetic fieldfor changing a spatial position of the first region responsive to themovement of the invasive medical instrument thereby changing amagnetization of the magnetic particles local to the invasive medicalinstrument with the first region substantially following the movement ofthe invasive medical instrument and substantially maintaining a positionof the first region with respect to the invasive medical instrument;detecting the change in magnetization of the magnetic particles in theexamination area affected by the change in the spatial position of thefirst region; and determining, by a processor, a spatial distribution ofthe magnetic particles in the examination area based on the detectedchange in magnetization of the magnetic particles.
 15. A system fordetermining spatial distribution of magnetic particles in an examinationarea, the arrangement comprising: an invasive medical instrument; afirst coil device having first and second coils configured to generate amagnetic field in the examination area including a low magnetic fieldstrength region and a higher magnetic field strength region; a secondcoil device configured to generate a spatially variable magnetic fieldin the examination area to change a spatial position of the low magneticfield strength region responsive to movement of the invasive medicalinstrument thereby changing a magnetization of the magnetic particleslocal to the invasive medical instrument with the low magnetic fieldstrength region substantially following movement of the invasive medicalinstrument and substantially maintaining a position of the low magneticfield strength region with respect to the invasive medical instrument; athird coil device configured to detect the change in the magnetizationof the magnetic particles in the examination area affected by the changein the spatial position of the first region; and a processor configuredto determine a spatial distribution of the magnetic particles in theexamination area, wherein the spatial distribution is determined basedon the detected change in magnetization of the magnetic particles,wherein at least one of the first, second and third coil devices arearranged on the invasive medical instrument.
 16. The system as claimedin claim 15, wherein the invasive medical instrument is a catheterhaving a tip and at least one of the first, second and third coildevices are arranged on or in a vicinity of the catheter tip.
 17. Thesystem as claimed in claim 16, wherein the invasive medical instrumenthas an opening located at the tip and the second coil device is arrangedon the invasive medical instrument and wherein the low magnetic fieldstrength region is located in front of the opening.
 18. The system asclaimed in claim 17, wherein the first coil device is arranged on theinvasive medical instrument and comprises at least two coils arrangedcoaxially one inside another each having a common axis that runssubstantially along an axis of the invasive medical instrument.